a mystic tan is a sunless-tanning booth where you get sprayed with a solution that produces a natural looking tan in seconds,and days later your tan will "oxidize" and you will be left with a natural looking tan that will last for days.
Marc: Where do I get mystic tan?
Betty: - Sunset tan on 48th!
Marc:Ding! Ding! Ding!
(0riginally from Ugly Betty S01E18)